Sơn Đoòng Cave (Hang Son Doong, “Mountain River cave” in Vietnamese) is a solutional cave in Phong Nha-Kẻ Bàng National Park, Bố Trạch District, Quảng Bình Province, Vietnam. As of 2009 it is the biggest known cave in the world, and is located near the Laos–Vietnam border. Inside is a large, fast-flowing underground river. It is formed in Carboniferous / Permian limestone.
Hang Son Doong was found by a local man named Hồ-Khanh in 1991. The whistling sound of wind and roar of fiery stream in the cave heard through the entrance as well as the steep descent prevented the local people to enter the cave. Only in 2009 did the cave become internationally known after a group of scientists from the British Cave Research Association, led by Howard and Deb Limbert, conducted a survey in Phong Nha-Kẻ Bàng from 10 to 14 April, 2009. Their progress was stopped by a large, 60 metres (200 ft) high calcite wall, which was named The Great Wall of Vietnam. It was traversed in 2010 when the group reached the end of cave passage.
According to the Limberts, the cave is five times larger than the Phong Nha Cave, previously considered the biggest cave in Vietnam. The biggest chamber of Hang Son Doong is more than five kilometres long, 200 metres high and 150 metres wide. With these dimensions, Hang Son Doong overtakes Deer Cave in Malaysia to take the title of the world’s largest cave. Cave contains some of the tallest known stalagmites in the world, which are up to 70 m tall. Behind the Great Wall of Vietnam were found cave pearls the size of baseballs, an abnormally large size.